1. Field of the Invention
The present invention concerns a method for automatically adjusting a radiation-gating diaphragm (having a number of adjustable diaphragm elements) for a subsequent x-ray exposure of an examination subject, wherein the individual diaphragm elements are respectively positioned such that they—considered in a projection lying in a detector plane—contact the contours of the acquisition subject or are arranged at a small distance therefrom. The invention also concerns an x-ray system with an x-ray source, an x-ray detector, a diaphragm (arranged in an x-ray beam path between the x-ray source and the x-ray detector) with a number of automatically adjustable diaphragm elements, and a diaphragm control device in order to position the individual diaphragm elements.
2. Description of the Prior Art
A diaphragm of the above type, known as a “depth diaphragm” or a “primary beam diaphragm” normally is located in the beam path between the x-ray source and the acquisition subject. It primarily has the object to allow only the acquisition subject to be irradiated, and not the surrounding areas. For example, in exposures of specific body parts or organs of a person, only tissue is irradiated that is necessary for the diagnosis or the planned intervention, such that the radiation dose for the subject is reduced. Moreover, given exposures in which, for example, the subject is a body part of a person, x-ray radiation is prevented from arriving unattenuated directly from the x-ray source (past the subject) to the detector. Such “direct radiation” can lead to image artifacts depending on the design of the detector. Thus, for example, due to scattering or due to transverse re-direction of the radiation in the detector glass, a lateral spatial expansion of the signal in the subject region can occur. This phenomenon can lead in an image intensifier to a phenomenon known as “low frequency drop”. Moreover, such direct radiation can lead to the individual structural elements of a planar image detector, assembled from a number of detector parts, respectively becoming visible at the Impact locations and interfering in the image. By the use of the depth diaphragm that covers (blocks) the irrelevant regions, so the beam precision is increased, and the image quality is improved.
Such a diaphragm can either completely gate the x-ray radiation or can be semi-transparent and attenuate the radiation. The first type of diaphragm has the advantage that no x-ray radiation whatsoever arrives in the irrelevant region. By contrast, the second type of diaphragm has the advantage that, although the regions located near the actual exposure subject will appear lighter in the image, to the same extent that the visibility is increased in the region of interest, but high-contrast objects (such as, for example, operating instruments) that are laterally introduced into the examination subject) are still visible. In both versions, an optimally good adaptation of the diaphragm to the respective examination subject is important for the proper functioning of such a diaphragm, so that the examination subject is not covered by the diaphragm plates, and the surrounding regions of no interest are covered to the extent possible.
In most conventional x-ray examination apparatuses, it is only possible to effect the adjustment of the diaphragm by hand, for example with the aid of a light-beam localizer. Moreover, there are x-ray systems in which it is possible to implement an automatic preadjustment using an organ program downloaded into a system control that before any manual adjustment, an approximate diaphragm adjustment to the region of interest is made. A disadvantage is that the actual position of the examination subject can vary significantly due to the positioning (for example of a patient) and variation in the size of a patient or of the respective examination subject. An ideal adjustment thus is not possible with these methods. In contrast to this, an exact adjustment of the depth diaphragm by fine adjustment by hand requires a relatively long time, which is counter to achieving an optimal workflow with short wait times for the patients.
German 35 00 812 describes an x-ray irradiation apparatus with a diaphragm of the above-described type, which has a number of diaphragm elements in the form of plates or lamellae that are positioned such that they abut the contours of the examination subject—viewed as a projection in the detector plane—at least at one point, meaning that the diaphragm elements projected from the x-ray source onto the detector plane abut the contours of the examination subject likewise projected from the x-ray source onto the detector plane. For this purpose, the apparatus has a placement device for the individual plates. The detector an x-ray image intensifier with a television camera connected thereto to generate video signals. Connected to the television camera is a special evaluation circuit which is designed such that specific image regions in the video signal are each associated with specific plates or lamellae. At the beginning of an exposure of an examination subject, the diaphragm is completely open. With the control signals acquired from the video signal, the individual plates are then controlled by the evaluation circuit such that they move toward one another and thus slowly close the diaphragm. Each individual plate is stopped as to its closing motion when a specific preselected brightness level is undershot in the portion of the video signal associated with the corresponding plate. This technique consequently requires that the subject be irradiated for a certain amount of time during the adjustment of the diaphragm. Moreover, the use of the technique is limited to x-ray detectors with a video camera and with a special evaluation circuit for the video signals.